[systemd-sonic-generator] Fix handling service files with additional fields under [Install] section

[stepanblyschak]

If encountered a line without RequiredBy or WantedBy the code passes uninitialized pointer to get_install_targets_from_line(). Where it can fail with segfault or silently pass randomly.

Why I did it

Uninitialized target_suffix is passed to get_install_targets_from_line() when other fields are present in [Install] section, like this:

root@sonic:/home/admin# systemctl cat ntpsec
...
[Install]
Alias=ntp.service
Alias=ntpd.service
WantedBy=multi-user.target

Work item tracking

• Microsoft ADO (number only):

How I did it

Initialize target_suffix with NULL, put an assert in get_install_targets_from_line(). Edited test to cover this scenario.

How to verify it

UT and on the switch.

Without the change - assert fails:

[       OK ] SsgFunctionTest.missing_file (2 ms)
[ RUN      ] SsgFunctionTest.insert_instance_number
[       OK ] SsgFunctionTest.insert_instance_number (2 ms)
[ RUN      ] SsgFunctionTest.get_num_of_asic
[       OK ] SsgFunctionTest.get_num_of_asic (2 ms)
[ RUN      ] SsgFunctionTest.get_unit_files
[       OK ] SsgFunctionTest.get_unit_files (2 ms)
[----------] 4 tests from SsgFunctionTest (8 ms total)

[----------] 4 tests from SsgMainTest
[ RUN      ] SsgMainTest.ssg_main_argv
Installation directory required as argument
[       OK ] SsgMainTest.ssg_main_argv (2 ms)
[ RUN      ] SsgMainTest.ssg_main_single_npu
ssg_test: systemd-sonic-generator.c:145: get_install_targets_from_line: Assertion `install_type' failed.
Aborted (core dumped)

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)